Re: Emitting JSON to file using COPY TO - Mailing list pgsql-hackers

Hi jian,

Thanks for keeping the patch set up to date with the master.

On Fri, Feb 6, 2026 at 11:26 AM jian he <jian.universality@gmail.com> wrote:
>
> On Wed, Feb 4, 2026 at 12:41 AM Florents Tselai
> <florents.tselai@gmail.com> wrote:
> >
> > I (and others I assume) would really like to see this in 19;
> > glancing at the thread above and in the test cases I see this is in good shape for comitter review.
> > No?
> >
> > If I were to add something  it would be an example in copy.sgml
> >   <para>
> >    When the <literal>FORCE_ARRAY</literal> option is enabled,
> >    the entire output is wrapped in a JSON array and individual rows are separated by commas:
> >    <programlisting>
> > COPY (SELECT id, name FROM users) TO STDOUT (FORMAT JSON, FORCE_ARRAY);
> >    </programlisting>
> >    <programlisting>
> > [
> > {"id": 1, "name": "Alice"}
> > ,{"id": 2, "name": "Bob"}
> > ,{"id": 3, "name": "Charlie"}
> > ]
> >    </programlisting>
> >   </para>
> >
>
> v23-0003-Add-option-force_array-for-COPY-JSON-FORMAT.patch
> I've added:
>
> +<para>
> +   When the <literal>FORCE_ARRAY</literal> option is enabled,
> +   the entire output is wrapped in a single JSON array with rows
> separated by commas:
> +<programlisting>
> +COPY (SELECT * FROM (VALUES(1),(2)) val(id)) TO STDOUT  (FORMAT JSON,
> FORCE_ARRAY);
> +</programlisting>
> +The output is as follows:
> +<screen>
> +[
> + {"id":1}
> +,{"id":2}
> +]
> +</screen>
> +</para>
> +
> +
>
> > Also, apologies if that has been discussed already,
> > is there a good reason why didn't we just go with a simple "WRAP_ARRAY" ?
> >
>
> I don’t have a particular preference.
> If the consensus is that WRAP_ARRAY is better than FORCE_ARRAY, we can
> change it accordingly.
>
>
> --
> jian
> https://www.enterprisedb.com/

Here are some comments on v23:

0001: The refactor looks straightforward to me. Introducing a format
field should make future extensions easier. One suggestion is that we
could add some helper macros around format, for example:

#define IS_FORMAT_CSV(format) (format == COPY_FORMAT_CSV)
#define IS_FORMAT_TEXT_LIKE(format) \
    (format == COPY_FORMAT_TEXT || format == COPY_FORMAT_CSV)

I think this would improve readability.

0002: Since you have moved the `CopyFormat enum` into 0001, the
following commit msg should be rephrased.

The CopyFormat enum was originally contributed by Joel Jacobson
joel@compiler.org, later refactored by Jian He to address various issues, and
further adapted by Junwang Zhao to support the newly introduced CopyToRoutine
struct (commit 2e4127b6d2).

- if (opts_out->format == COPY_FORMAT_BINARY && opts_out->delim)
- ereport(ERROR,
- (errcode(ERRCODE_SYNTAX_ERROR),
- /*- translator: %s is the name of a COPY option, e.g. ON_ERROR */
- errmsg("cannot specify %s in BINARY mode", "DELIMITER")));
+ if (opts_out->delim)
+ {
+ if (opts_out->format == COPY_FORMAT_BINARY)
+ ereport(ERROR,
+ errcode(ERRCODE_SYNTAX_ERROR),
+ /*- translator: %s is the name of a COPY option, e.g. ON_ERROR */
+ errmsg("cannot specify %s in BINARY mode", "DELIMITER"));
+ else if (opts_out->format == COPY_FORMAT_JSON)
+ ereport(ERROR,
+ errcode(ERRCODE_SYNTAX_ERROR),
+ errmsg("cannot specify %s in JSON mode", "DELIMITER"));
+ }

Can we add a function that converts CopyFormat to a string? Treating
CopyFormat as %s in error messages would make the code shorter.
However, I'm not sure whether this aligns with translation
conventions, correct me if I'm wrong.

- * CSV and text formats share the same TextLike routines except for the
+ * CSV and text, json formats share the same TextLike routines except for the

I'd suggest rewording to `CSV, text and json ...`. The same applied to
other parts in this patch.

0003: The commit message includes some changes(adapt the newly
introduced CopyToRoutine) that actually belong to 0002; it would be
better to remove them from this commit.

+ if (cstate->json_row_delim_needed && cstate->opts.force_array)
+ CopySendChar(cstate, ',');
+ else if (cstate->opts.force_array)
+ {
+ /* first row needs no delimiter */
+ CopySendChar(cstate, ' ');
+ cstate->json_row_delim_needed = true;
+ }

can we do this:

if (cstate->opts.force_array)
{
    if (cstate->json_row_delim_needed)
        CopySendChar(cstate, ',');
    else
    {
        /* first row needs no delimiter */
        CopySendChar(cstate, ' ');
        cstate->json_row_delim_needed = true;
    }
}


--
Regards
Junwang Zhao